In rotating machines of large dimensions, such as vertically arranged hydrogenerators for example, special problems occur when mounting the rotors in bearings, these problems being caused in particular by the extreme ratio between the external dimensions, which usually amount to several meters, and the bearing play, which is within the range of tenths of a mm. The rotors of such hydrogenerators are normally fixed with regard to an axis of rotation by means of two radial or guide bearings at a distance from one another. The weight and other axial forces, originating in particular from the water turbine, are absorbed by at least one axial or supporting bearing, which is usually combined with one of the radial bearings to form a combined axial and radial bearing and is accommodated in a common bearing body. The bearing bodies are the central bodies of a star-shaped bearing bracket or star-shaped supporting bracket or star-shaped guide bracket which is formed by supporting arms starting from the bearing body in the radial direction. The supporting arms, for absorbing axial forces, are supported with their free outer ends in the axial direction either on the foundation, which is normally made of concrete and concentrically surrounds the generator, or on the stator of the generator. Examples of such star-shaped supporting brackets or star-shaped guide brackets are described in CH-A5-578 708 mentioned at the beginning or in U.S. Pat. No. 4,258,280 corresponding thereto or in CH-A5-583 469. An example of a combined axial and radial bearing or supporting and guide bearing is found in FIG. 1 of EP-A1-0 586 861.
In order to also be able to absorb radial forces which are caused, for example, by unbalance in the rotor or by interactions between rotor and stator, the supporting arms in the prior art have been extended in the radial direction up to the surrounding concrete wall and firmly screwed to metal elements embedded there. As a result, high rigidity of the star-shaped supporting bracket was achieved, this rigidity having been determined essentially by the rigidity of the arms stressed in compression in the longitudinal direction. A disadvantage with this arrangement, however, is that very high compressive forces acting in the radial direction are produced during a thermal expansion of the arms of the star-shaped supporting bracket, and these compressive forces, especially in the case of star-shaped supporting brackets arranged above the machine, cannot be readily absorbed by the concrete wall, which is rather thin there. In addition, such a rigid construction introduces large radial forces into the bearing housing. Said radial forces may affect the bearing play due to resulting eccentricity or deformation of the bearing housing and thus put the operability of the bearing at risk in the extreme case. It has therefore been proposed in CH-A5-583 469 already mentioned to mutually stiffen the arms of a star-shaped supporting bracket and to secure their free ends in the tangential direction in the concrete wall by prestressed rolling bodies, while they are mounted such as to be displaceable in the radial direction.
On the other hand, CH-A5-578 708 mentioned at the beginning has adopted another method of solving the problem associated with the thermal expansion in the case of star-shaped supporting brackets or star-shaped guide brackets: used here for absorbing radial forces are connecting elements which are arranged tangentially to an imaginary cylinder coaxial to the axis of rotation and—in a pure radial or guide bearing—form the arms of the star-shaped bearing bracket (see FIG. 2 of CH-A5-578 708) or—in a combined axial and radial bearing—run between the ends of the (radial) supporting arms and the concrete wall (shown in FIG. 6 of CH-A5-578 708). In more recent hydrogenerators (as are used, for example, in the Bieudron hydroelectric plant, Switzerland; in this respect also see the applicant's brochure “Hydro-generators with oblique elements. Top technology from ALSTOM power”, September 2000), the supporting arms of the combined supporting and guide bearings including the connecting elements have a tangential orientation relative to the concrete wall in a continuous robust welded construction.
The tangentially oriented arrangement of the connecting elements or supporting arms achieves the effect that thermal expansions are converted into uncritical rotations of the bearing body about the machine axis. The mode of operation of the tangentially oriented connecting elements or oblique spokes is described in detail in the publication Brown Boveri Mitteilungen 2, volume 67, pages 108-116, February 1980.
However, a disadvantage with the conventional star-shaped supporting brackets having tangentially oriented connecting elements is the direct linkage between the actual supporting arms, which absorb the axial forces and pass them into the foundation, and the tangential connecting elements, which convert the radial forces into a rotation and are responsible for the centering of the bearing body. This direct linkage leads on the one hand to heavy constructions of the star-shaped bearing bracket, which require a lot of material, and prevents on the other hand separate optimization of the construction elements responsible for the axial and radial forces. In addition, asymmetrical deformations, as may occur due to changes in the foundation or due to powerful electromagnetic fields, cannot be readily compensated for.